Muscle use during dynamic knee extension: implication for perfusion and metabolism.

Dynamic one-legged knee extension (DKE) is commonly used to examine physiological responses to "aerobic" exercise. Muscle blood flow during DKE is often expressed relative to quadriceps femoris muscle mass irrespective of work rate. This is contrary to the notion that increased force is achieved by recruitment in large muscles. The purpose of this study, therefore, was to determine muscle use during DKE. Six subjects had magnetic resonance images taken of their quadriceps femoris before and after 4 min of DKE at 20 and 40 W. Muscle use was determined by shifts in T2. The cross-sectional area of quadriceps femoris that had an elevated T2 was 16 +/- 1% (mean +/- SE) preexercise, and 54 +/- 5 and 94 +/- 4% after 20- and 40-W DKE, respectively. Volume of quadriceps femoris increased 11.4 +/- 0. 2% (P = 0.006), from 2,230 +/- 233 cm3 before exercise to 2,473 +/- 232 cm3 after 40-W DKE. Extrapolation of these data indicates that 1, 301 +/- 111 cm3 of quadriceps femoris were engaged during 20-W DKE compared with 2,292 +/- 154 cm3 during 40-W DKE. By using muscle blood flow data for submaximal DKE at 20 W [P. Andersen and B. Saltin. J. Physiol. (Lond.) 366: 233-249, 1985; and L. B. Rowell, B. Saltin, B. Kiens, and N. J. Christensen. Am. J. Physiol. 251 (Heart Circ. Physiol. 20): H1038-H1044, 1986] and estimating muscle use in those studies from our data (total muscle mass x 0.54), extrapolated blood flow to active muscle (263 and 278 ml . min-1 . 100 g-1, respectively) is comparable to that obtained during peak aerobic DKE when expressed relative to total muscle mass (243 and 250 ml . min-1 . 100 g-1, respectively). These findings indicate that increased power during aerobic DKE is achieved by recruitment. Additionally, they suggest that blood flow to the active quadriceps femoris muscle does not increase with increases in submaximal work rate but instead is maximal to support aerobic metabolism. Thus increases in muscle blood flow are directed to newly recruited muscle, not to increased perfusion of muscle already engaged.